Purification of alloys



Unite States Patent 3,309,194 PUREFICATION 0F ALLOYS Edward J. Dunn, Jr., Library, Pa., assignor to Westinghouse Electric Corporation, Pittsburgh, Pa., :1 corporation of Pennsylvania No Drawing. Filed Dec. 24, 1964, Ser. No. 421,114 7 Claims. (Cl. 75-52) This invention is directed to the purification of ironbase and nickel-base alloys.

In certain iron-base and nickel-base alloys, as, for example, some of the soft magnetic alloys, it is highly desirable that the residual levels of sulfur, oxygen and carbon be as low as possible. The copending application Serial No. 242,214, filed December 4, 1962, and entitled, Purification of Alloys, by Edward J. Dunn and Donald M. Kelman, now US. Patent No. 3,212,881, describes a method for reducing the sulfur and oxygen contents to remarkably low levels during the melting process. It is also desirable to reduce the carbon content of such alloys to levels below about 0.01%, by weight, of carbon.

In the prior art it is known that the carbon content of iron-base and nickel-base alloys can be reduced substantially by several techniques, including appropriate melting procedures, or by treatment either of the alloys or of the individual raw materials while in the solid state to reduce their carbon content. It will be understood that the most convenient and economical of the above methods is the reduction of carbon content during melting of the alloy, but that the known melting procedures are incapable of reducing the carbon content of the alloys to less than 0.01%, by weight, of carbon, while at the same time obtaining a low oxygen residual.

Accordingly, where low-carbon material with a low oxygen residual has been required in the past this has been obtained generally by the solid state technique in which the alloy in solid form is exposed in a furnace to a hydrogen atmosphere at elevated temperature so that the hydrogen diffuses into the alloy body and combines with the carbon to evolve methane (CH or other hydrocarbon gas which then enters the furnace atmosphere and is carried away. However, some alloys cannot be heat treated in the finished shape to remove carbon because this would adversely affect their magnetic properties. On occasion,

low-carbon material has also been obtained by individually treating raw materials in the solid state with hydrogen as described above to remove carbon prior to melting the alloy, but this is both inconvenient and relatively expensive.

It is apparent, consequently, that a need exists for a melting technique capable of producing low carbon alloys, and this also extends to alloys with low carbon as Well as having low sulfur and oxygen residuals.

Accordingly, it is an object of the invention to provide a process for producing alloys having low carbon content and also having low sulfur and oxygen content.

It is a further object of this invention to provide a method for reducing the carbon content of alloys to low levels as a further step in a process capable of reducing the sulfur and oxygen content of the alloys to low levels.

It is another object of the invention to provide a proc ess capable of reducing the carbon content of iron-base and nickel-base alloys to the low level of less than 0.01%, by weight, while the alloys are in the molten state.

Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter. For a better understanding of the nature and objects of this invention, reference should be had to the following detailed description:

Broadly, this invention is concerned with an improvement in the process for employing elemental calcium as an addition during the melting of alloys to reduce the sulfur and oxygen contents thereof, whereby the carbon contents of the treated alloys may be also reduced. More specifically, the process of this invention is an improvement over the process set forth in the above-mentioned application Serial No. 242,214 wherein a process is de scribed in which an iron-base or nickel-base molten alloy is provided with a lime slag cover capable of absorbing calcium-sulfur compounds and preventing their reversion to :the bath. This slag consists primarily of lime (C-aO) but may also include fiuorspar (CaF in amounts up to 10% by weight or even more. From 0.5% to 2% by weight, and preferably from 0.6% to 1%, by weight of elemental calcium is added to the bath whereby the oxygen and sulfur in the bath are reduced to low levels, and thereafter the alloy heat is poured. If desired to improve forgeability, the residual calcium, amounting to perhaps 0.03%, by weight, may then be reduced in amount by one or more of the methods comprising (1) flotation (holding the molten alloy at temperature to permit the calcium to rise to the surface and there to combine with the slag), (2) oxidation (exposing the molten alloy to an oxidizing atmosphere to oxidize the calcium whereupon the resulting calcium oxide will rise to the surface of the alloy), (3) vacuum treatment (exposing molten alloy to vacuum whereby calcium metal is removed by vaporization).

In the process of the present invention the above described process is carried out with additional steps to reduce the carbon content of the iron-base or nickel-base alloys which are being treated. To obtain the desired reduction in carbon content the temperature of the molten alloy is lowered to a temperature below about 1755 K. and then the molten alloy is thoroughly exposed to an oxidizing atmosphere.

While the generally-accepted method for reducing the carbon content of iron-base and nickel-base alloys is through the reaction of carbon with oxygen (the oxygen source may either be iron oxide or oxygen gas), this method is not capable of reducing the carbon content much below the level of 0.02% to 0.03% by weight of,

carbon. In order to reduce the carbon content to lower limits it is necessary to resort to other more drastic and costly methods. I

The reactions which come into play as a result of the addition of calcium to a molten alloy of the iron-base or nickel-base type containing oxygen, sulfur and carbon may be set forth as follows:

By the techniques described in the copending application Serial No. 242,214, the reactions indicated by the first two of the above equations have been made to effect a reduction in both the sulfur and oxygen content of the alloys to extremely low levels. This reduction of sulfur and oxygen content is a prerequisite to obtaining the third reaction listed above in which calcium and carbon combine to form calcium carbide, because calcium reacts with oxygen or sulfur in preference to carbon. Further, temperatures above about 1755 K. favor dissociation of calcium carbide, While the calcium-carbon reaction is favored below that temperature. At about 1755 K. the dissociation and formation reactions are in equilibrium. Since the reactions set forth in the prior application were carried out at temperatures in excess of 1750 K. the calcium-carbon reaction was not favored and no substantial reduction of carbon was obtained.

It has been found, and it is on this discovery that this invention is primarily based, that in order for the calcium-carbon reaction to occur, the temperature of the molten alloy must be reduced to a point below the equilibrium temperature of about 1755 K. However, while the desired reaction will occur over the whole range of temperatures extending from the melting point of the particular alloy up to temperatures below about 1755 K., to obtain a substantial driving force for the reaction, the temperature of the molten alloy should be to Kelvin below about 1755 K., say 1740" K. or lower. 1700 -t25 K. has been found to be a good working temperature. Once the calcium-carbon reaction has 'been made to occur and the calcium carbide is present in substantial amounts, the fourth reaction listed above can be obtained if sufficient oxygen is present. The products of this reaction, C210 and CO are readily eliminated, the first floating to join the slag cover and the second rising and leaving the molten alloy as a gas. Accordingly, then, one effective means for obtaining both the temperature reduction and the exposure to oxygen is a reladling technique in which the molten alloy is poured in air from the furnace to a ladle, permitted to cool to the desired temperature below about 1755 K. (Kelvin) in the ladle, and then is poured back into the furnace. In some cases the heat may be poured into a mold directly from the ladle. It should be understood that the reladling practice facilitates good mixing of the alloy with oxygen in the air at a temperature low enough to permit the formation of calcium carbide. This oxidation step may raise the level of oxygen once again in the molten alloy and it may therefore be desirable to make a second calcium addition to reduce the oxygen content, if sufficient calcium does not remain in the molten alloy from the initial addition to combine with the oxygen. This second calcium addition may be made in amounts of from 0.6% to 1%, by weight.

The above technique has been practiced successfully on several heats of iron-base and nickel-base alloys as listed below in the table.

TABLE I.-EFFEOT OF CALCIUM PLUS RELADLE Nominal 29 Nickel, 17 Cobalt, balance Iron. 2 Composition 48 Nickel, balance Iron.

It will be noted that in the above alloys, which normally have about 0.02% carbon, by weight, by this process the level of carbon has been reduced to a few thousandths of a percent by weight of carbon. Further, the sulfur and oxygen contents of these alloys, normally at a level of .01 to .015% by weight, is desirably low.

It must be understood that this technique is not one which can be universally practiced on all alloys in which it might be considered desirable to obtain a low carbon content. This technique is only useful in those alloys Which have a melting point of below 1500 C. The alloys of Table I thus have melting points of about 1450 C. or lower. When this practice was attempted on an alloy having a melting point of 1500 C., the molten alloy began to solidify at a temperature about 1750 K. and no substantial reduction in carbon content was achieved.

The inventive principles embodying the above description may obviously be incorporated in modified processes by those skilled in the art without departing from the spirit and scope of this invention, and it is intended that the description be interpreted as illustrative and not in a limiting sense.

I claim as my invention:

1. In a process for producing an alloy having low sulfur, oxygen and carbon content wherein the alloy is selected from the group consisting of iron-base and nickelbase alloys, the steps comprising adding to the molten alloy from 0.5% to 2%, by weight, of calcium, whereby calcium compounds of sulfur and oxygen are formed and permitted to segregate and thus effect a reduction in the sulfur and oxygen content of the molten alloy, lowering the temperature of the alloy to below about 1755 K., but above the melting point of the alloy, while free calcium is present in the alloy and introducing oxygen into said alloy while at said temperature whereby a reaction between the calcium, carbon and oxygen occurs and results in the reduction of the carbon content of the alloy, and separating and removing free calcium and calcium compounds from the molten alloy.

2. In a process for producing low sulfur, oxygen and carbon contents in alloys selected from the group consisting of iron-base and nickel-base alloys having melting points below 1500 C. wherein the sulfur and oxygen content of the molten alloy has been reduced to a low level by a calcium addition, the steps comprising lowering the temperature of the molten alloy to below about 1755 K. but above the melting point of the alloy, thoroughly exposing said molten alloy while at said temperature to an oxidizing atmosphere whereby the carbon content of the alloy is reduced, making another addition of from 0.6% to 1%, by weight, of calcium to the molten alloy to reduce the amount of oxygen in the alloy and thereafter pouring the heat.

3. The process of claim 2 wherein any excess calcium remaining in the molten alloy prior to pouring is removed 'by exposing the molten alloy to a vacuum environment.

4. In a process for producing low sulfur, oxygen, and carbon contents in alloys selected from the group consisting of iron-base and nickel-base alloys, the steps comprising, providing the molten alloy with a lime slag cover capable of absorbing calcium and calcium-sulfur compounds and preventing their reversion to the bath, adding from 0.5% to 2%, by weight, of elemental calcium to the bath whereby the sulfur and oxygen in the bath are reduced to low levels, lowering the temperature of the molten alloy to below about 1755" K. but above the melting point of the alloy and thoroughly exposing said molten alloy while at said temperature to an oxidizing atmosphere whereby the carbon content of the alloy is re duced, holding the melt under the slag cover for a time sufiicient to permit the calcium to rise through the molten metal to the slag cover and thereby reduce the calcium content of the molten metal, and thereafter pouring the metal.

5. The process of claim 4 wherein the molten alloy is exposed to a vacuum environment subsequent to the oxidation step to remove excess calcium.

6. In a process for obtaining low sulfur, oxygen and carbon contents in alloys selected from the group consisting of iron-base and nickel-base alloys having melting points below 1500 C., the steps comprising, providing the molten alloy with a lime slag cover capable of absorbing calcium and calcium-sulfur compounds and preventing their reversion to the bath, adding from 0.5% to 2%, by

weight, of elemental calcium to the bath whereby the sulfur and oxygen in the bath are reduced to low levels, lowering the temperature of the molten alloy to 1700:25" K. and thoroughly exposing said molten alloy while at said temperature to an oxidizing atmosphere whereby the carbon content thereof is reduced, making another addition of from 0.6% to 1% by weight of calcium to the bath to lower the oxygen content thereof, holding the melt under the slag cover for a time sufiicient to permit the calcium and calcium oxide to rise through the metal to the slag cover and thereby reduce the calcium content of the molten metal, and thereafter pouring the metal.

7. The process of claim 6 wherein the molten alloy is exposed to a vacuum environment prior to pouring to further reduce excess calcium.

References Cited by the Examiner UNITED STATES PATENTS BENJAMIN HENKIN, Primary Examiner. DAVID L. RECK, Examiner. 

1. IN A PROCESS FOR PRODUCING AN ALLOY HAVING LOW SULFUR, OXYGEN AND CARBON CONTENT WHEREIN THE ALLOY IS SELECTED FROM THE GROUP CONSISTING OF IRON-BASE AND NICKELBASE ALLOYS, THE STEPS COMPRISING ADDING TO THE MOLTEN ALLOY FROM 0.5% TO 2%, BY WEIGHT, OF CALCIUM, WHEREBY CALCIUM COMPOUNDS OF SULFUR AND OXYGEN ARE FORMED AND PERMITTED TO SEGREGATE AND THUS EFFECT A REDUCTION IN THE SULFUR AND OXYGEN CONTENT OF THE MOLTEN ALLOY, LOWERING THE TEMPERATURE OF THE ALLOY TO BELOW ABOUT 1755*K., BUT ABOVE THE MELTING POINT OF THE ALLOY, WHILE FREE CALCIUM IS PRESENT IN THE ALLOY AND INTRODUCING OXYGEN INTO SAID ALLOY WHILE AT SAID TEMPERATURE WHEREBY A REACTION BETWEEN THE CALCIUM, CARBON AND OXYGEN OCCURS AND RESULTS IN THE REDUCTION OF THE CARBON CONTENT OF THE ALLOY, AND SEPARATING AND REMOVING FREE CALCIUM AND CALCIUM COMPOUNDS FROM THE MOLTEN ALLOY. 